Sen. Tom Cotton, the hawkish upstart who’s already made waves onthe Iran nuclear deal and government surveillance programs, is now leading a new rebellion against a bipartisan effort to overhaul the criminal justice system — hoping to torpedo one of the few pieces of major legislation that could pass Congress in President Barack Obama’s final year.

GOP tensions over a bill that would effectively loosen some mandatory minimum sentences spilled over during a party lunch last week, when Cotton (R-Ark.), the outspoken Senate freshman, lobbied his colleagues heavily against the legislation, according to people familiar with the closed-door conversation. The measure passed the Senate Judiciary Committee last fall with bipartisan support.

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“It would be very dangerous and unwise to proceed with the Senate Judiciary bill, which would lead to the release of thousands of violent felons,” Cotton said later in an interview with POLITICO. “I think it’s no surprise that Republicans are divided on this question … [but] I don’t think any Republicans want legislation that is going to let out violent felons, which this bill would do.”

Cotton isn’t alone. Other Senate Republicans, including Sens. Jim Risch of Idaho and David Perdue of Georgia, also registered their strong opposition during the lunch, even as Sen. Mike Lee (R-Utah) vigorously defended the bill, which he helped negotiate. Risch stressed this message, according to one Republican source: Shouldn’t the GOP be a party of law and order?

Risch declined to elaborate on his concerns over the bill, saying he was displeased that his private remarks made during a party lunch were made public. But the deepening Republican split over reforming key elements of the criminal justice system — an effort years in the making that has been powered by an influential right-left coalition — may imperil whether Senate Majority Leader Mitch McConnell ultimately will take up the measure later in this election year.

Conservatives opposing the legislation are coalescing around Cotton’s view — despite strong pushback from bill supporters — that the measure could lead to the early release of people convicted and imprisoned for violent crimes. Sen. Ted Cruz (R-Texas), once a supporter of easing mandatory minimums for nonviolent drug offenders, has also made this argument. And there’s stiff resistance in pockets of the Republican Party to do anything that might erode its tough-on-crime reputation.

Backers of the bill say their changes to sentencing laws merely allow qualifying inmates to have their cases revisited by the same judge and prosecutor who landed them in prison. The judge would then have the discretion to hand down a reduced sentence.

“It’s not true,” said Senate Majority Whip John Cornyn (R-Texas) of opponents’ insistence that violent criminals could be freed under the sentencing reforms. “I’d say, please read the bill and listen to people like [former Attorney General] Michael Mukasey, who makes the point, which is a critical point, that there’s no get-out-of-jail-free card.”

But that perception, hardening among conservatives, is a serious obstacle for supporters of the bill like Cornyn, who as the Senate’s second-ranking Republican is the most influential GOP backer of the criminal-justice measure. And last week, McConnell — who is often hesitant to press ahead on issues that divide his 54-member conference — indicated a breather of sorts on the bill, saying GOP senators would take some time to get educated on the measure.

Those comments discouraged some supporters, since any major pause could spell doom for the bill this year. In a couple of months, the GOP-led Congress will turn its attention to its top legislative priority — budget and appropriations bills — while individual lawmakers shift into full campaign mode.

“Members of the Judiciary Committee have been deeply involved on that issue, the rest of us have not,” McConnell told reporters of criminal justice reform. “So we’re going to be working through the process of bringing everybody in the Republican Conference up to speed on this very important issue, and we’re going to do that before any decision is made about floor time.”

The criminal justice overhaul isn’t limited to sentencing reforms. The measure also includes reforms to the prison system championed by Cornyn and Sen. Sheldon Whitehouse (D-R.I.) — changes that Cotton said he supports. And overhaul efforts also are complicated by the issue of so-called mens rea reform, with House Republicans and some GOP senators — including Orrin Hatch of Utah, the most senior Senate Republican — demanding changes to rules governing criminal intent.

But the sentencing changes are triggering the biggest — and most vivid — rift among Republicans. Cotton and other Republicans pointed to a triple murder earlier this month in Columbus, Ohio, in which a man is accused of killing an ex-girlfriend and two of her children. The suspect, Wendell Callahan, had his prison sentence on drug charges reduced twice for a total of more than four years, according to The Columbus Dispatch.

“As a Republican Party, we’re going to have to have a conversation about it,” Cotton said in the interview. “But I think, ultimately, a majority of Republicans, like a majority of Americans, don’t want to let violent felons out of prison.”

Supporters note that easing those mandatory minimum sentences would affect just about 3,900 prisoners per year, according to one GOP aide. On top of those inmates, the legislation would apply retroactively to nearly 4,800 federal prisoners, but only under certain circumstances.

Backers also point out that the bill actually toughens some mandatory minimum sentences, namely for drug crimes committed by people with prior serious violent felony convictions.

“We can no longer ignore the cost of our prison population,” Vermont Sen. Patrick Leahy, the Judiciary Committee’s top Democrat, said at a hearing on the issue last week. “We must not turn our backs on the families that are being torn apart by needlessly harsh prison sentences that do not make us safer.”

High-ranking lawenforcement officials, such as Mukasey, who served as attorney general under President George W. Bush, and ex-FBI Director Louis Freeh have put their weight behind the bipartisan criminal justice measure. But opponents are marshaling other law-enforcement heavyweights against the bill.

In December, former Attorney General John Ashcroft — who, like Mukasey, served in the Bush administration — and Rudy Giuliani, who presided over a steep decline in crime as mayor of New York City in the 1990s, wrote to congressional leaders with concerns over the Senate bill. They warned that the legislation would bring “significant risks to public safety” and raised worries about the bill’s retroactive provisions.

“Our system of justice is not broken,” they wrote in the Dec. 10 letter to McConnell and Minority Leader Harry Reid (D-Nev.). “Mandatory minimums and proactive law-enforcement measures have caused a dramatic reduction in crime over the past 25 years, an achievement we cannot afford to give back.”